The Federal Aviation Administration is warning pilots of some Boeing 737s to be on the lookout for problems with a sensor on the aircraft. The emergency directive echoes a safety bulletin from Boeing itself in the wake of last week's Lion Air plane crash in Indonesia that killed all 189 people onboard. Investigators say that sensor appears to have malfunctioned, as NPR's David Schaper reports.

DAVID SCHAPER, BYLINE: Commercial airline crashes are quite rare these days. And the crash of the Lion Air 737 MAX jet into the Java Sea on October 29 is even more unexpected.

TODD CURTIS: This was an unusual circumstance. This is a newly certified aircraft, certified just in 2017.

SCHAPER: Todd Curtis is a former safety engineer at Boeing and now heads the airsafe.com Foundation. And he says the new plane that crashed, in service for just two months, was an updated version of Boeing's most popular jet.

CURTIS: There are no really radically different things about the way this aircraft was designed. Yes, it had more efficient engines and some other changes and some cockpit changes.

SCHAPER: Indonesian investigators say one of the plane's black boxes, the flight data recorder, indicates that the plane had a malfunctioning air speed indicator on its last four flights. And on the last two flights, it had problems with what's called the angle of attack sensor.

CURTIS: Well, the angle of attack sensor tells the flight crew and also the flight control system what angle the airstream is going over the wings.

SCHAPER: Curtis says that's critical because if the angle of attack is too high, the wings cannot generate enough lift to keep the plane airborne. But on the Lion Air 737, one of the sensors was providing the computer system and the pilots with erroneous information about the angle. Anthony Brickhouse is a former NTSB investigator who now teaches aviation safety at Embry-Riddle Aeronautical University in Florida.

ANTHONY BRICKHOUSE: And from what we know so far about the Lion Air situation is that the aircraft entered basically what would be considered an uncommanded nose-over-type maneuver.

SCHAPER: Indonesia's National Transportation Safety Committee, which is investigating the crash, found that the same thing happened the day before. The plane went into a sudden nosedive shortly after takeoff. But the pilots recovered, and after landing, the sensor was replaced.

Why it happened again on the fatal flight and how the malfunctioning sensor may have contributed to the crash are still not known. And Brickhouse notes that since the cockpit voice recorder has not yet been recovered, investigators do not yet know how the flight crew reacted to the sensor failure. But Brickhouse says all pilots should be ready for any such failure.

BRICKHOUSE: Pilots are professionally trained over and over and over again to respond to different situations.

SCHAPER: The emergency order from the FAA does not suggest 737 MAX planes are unsafe, nor does it warn that the sensor failure caused the Indonesia crash.

BRICKHOUSE: What this airworthiness directive is doing is basically putting airlines on notice who are flying the 737 MAX to definitely, you know, take a little extra caution in reviewing their procedures and following procedures that are already published.

SCHAPER: The FAA order covers 45 Boeing 737 MAX planes operated in the U.S. by several airlines, including Southwest, United and American. The new 737 MAX 8 and MAX 9 are extremely popular. Airlines globally have ordered more than 4,500 of them, and Boeing has had some production delays and kinks in its supply chain while trying to meet that hefty demand. But investigators say there is no evidence thus far that any such problems contributed to the flawed sensors or to the crash. David Schaper, NPR News.